71X - avoid RecoMET/METProducers DAS errors

This PR tries to get rid of the repeated (and annoying) problems from DAS queries in integration tests like e.g. https://github.com/cms-sw/cmssw/pull/3717#issuecomment-43599544
The errors appear when DAS is used to determine input files with `pickRelValInputFiles`, but seem unrelated to that script. The DAS reply contains usually messages like
```
[{u\'reason\': u\'HTTPError, system=phedex, url=https://cmsweb.cern.ch/phedex/datasvc/xml/prod/fileReplicas, args={"dataset": "/RelValTTbar_13/CMSSW_7_1_0_pre2-PU50ns_POSTLS170_V4-v2/GEN-SIM-RECO"}, headers={"Accept": "application/xml;text/xml", "User-Agent": "das-server/2.6.1::python/2.6"}\', u\'ts\': 1400572838.24, u\'error\': u\'Unable to contact phedex data-service.\'}]
```

This PR hides the problem rather than fixing it.
@davidlange6 : Maybe we should discuss in the ORP, if we want that - but I am rather sure, we do for CMSSW tests.

P.S.:
I did not see problems with the DAS queries in my local tests.